Ground surface hardness refers to a measure of the deceleration of an object upon impact with the surface. The units of deceleration can be expressed as G, which is the ratio of the magnitude of deceleration during impact to the acceleration of gravity. The maximum value of G encountered during impact is sometimes referred to as GMAX.
The hardness of an athletic field can be a factor to consider in evaluating the safety of the athletic field. In some sports, such as football, contact with the ground surface is likely. Knowledge of the hardness of the field is useful in determining the relative safety of the field, and in comparing the hardness of the field with the hardness of other fields.
On the other hand, it is sometimes desirable to have a relatively hard ground surface. On a putting green, for example, a harder ground surface is less prone to damage when hit by golf balls, and can require less water for irrigation.